The purpose of this study is to assess the role of maternal hematocrit/hemoglobin measures in the occurrence of preterm birth. A recent study by Lieberman et al. (New England Journal of Medicine 1987; 317:343-8) concluded that differences in hematocrit levels between different ethnic groups may account for the well known but otherwise unexplained differences in the risk of preterm birth between the ethnic groups. One phase of this project analyzed prospectively collected hematocrit data from the Collaborative Perinatal Project. The second phase of this project obtained additional data from a subset of women in the Kaiser-Permanente Birth Defects Study to assess the role of maternal hematocrit/ hemoglobin measures in the occurrence of preterm birth.